Apparatus for and method of optical component alignment

ABSTRACT

Apparatus for and method of aligning optical components such as beam splitters in an optical pulse stretcher in which a landing spot of a beam which has traversed a portion of the optical beam splitter and a coincident landing spot of a beam split from a retroreflected input beam are made to align on a target spot. Also disclosed is an apparatus and method for aligning the retroreflector to facilitate proper beam alignment. A fluorescent material may be used to render a beam landing spot visible.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Application No. 63/126,067 filed Dec. 16, 2020, titled APPARATUS FOR AND METHOD OF OPTICAL COMPONENT ALIGNMENT FIELD, which is incorporated herein in its entirety by reference.

FIELD

The present disclosure relates to systems and methods for aligning optical components for use, for example, in a lithographic apparatus, and particularly to components in optical pulse stretchers useful for lengthening the pulse of the output of a laser source.

BACKGROUND

A lithographic apparatus applies a desired pattern onto a substrate such as a wafer of semiconductor material, usually onto a target portion of the substrate. A patterning device, such as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the wafer. Transfer of the pattern is typically accomplished by imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain adjacent target portions that are successively patterned.

Lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or opposite this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. Herein, for the sake of simplicity, both steppers and scanners will be referred to simply as scanners.

The light source used to illuminate the pattern and project it onto the substrate can be of any one of a number of configurations. Deep ultraviolet excimer lasers commonly used in lithography systems include the krypton fluoride (KrF) laser at 248 nm wavelength and the argon fluoride (ArF) laser at 193 nm wavelength. The laser source can include an optical pulse stretcher for lengthening the pulse of the output of a high power gas discharge laser system.

Newer requirements for lithography scanner performance necessitate a longer pulse length, measured in terms of the time integral square (TIS) of the pulse length. For example, improving the Edge Placement Error (EPE) of a chip feature requires a longer TIS. An optical pulse stretcher (OPuS) is used to stretch the pulses to achieve the desired TIS. Increasing TIS requires a larger OPuS. Increasing the size of the OPuS makes it even more critical that the components of the OPuS are in proper optical alignment.

A conventional method of aligning the components of an OPuS entails opening the OPuS enclosure and physically positioning a target card near the optical surface of a component being aligned. The incoming beam is then aligned based on the landing position (or footprint) of the beam of the target card. The target card is then shifted to the next optical component requiring alignment and so on until all of the optical components requiring alignment have been properly aligned. As used herein, “ray” and “beam” share the same meaning.

This alignment process, which may be referred to as open beam paper target alignment, requires opening up the sealed laser enclosure for card placement. It entails the risks of open beam operation and must be executed very carefully to avoid exposing the skin of the individual performing the alignment to UV radiation. It is also time consuming and requires an extensive amount of manual operation. It exposes the optical surfaces being aligned to contamination and can result in a reduction of the optical life of critical optical components. It is also difficult to achieve accurate alignment due to the complexity of establishing the beam alignment target and subjective judgment of beam position.

The alignment process also entails aligning a beam splitter in the OPuS. This process entails several steps, some of which may need to be repeated if the beam is caused to “fall off of” one of the mirrors, i.e., miss the mirror, during the alignment process. This can greatly increase the time required to perform an alignment.

It is in this context that the need for the disclosed subject matter arises.

SUMMARY

The following presents a concise summary of one or more embodiments in order to provide a basic understanding of the embodiments. This summary is not an extensive overview of all contemplated embodiments and is not intended to identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later.

According to an aspect of an embodiment, a camera system is used to monitor the beam position on OPuS components such as mirrors to facilitate proper beam alignment. In some embodiments an image integration optical system is used to integrate images from multiple optical features such as from both left mirror bank and right mirror bank to present the images simultaneously to the camera system. This simplifies the design and avoids the cost of having multiple cameras to inspect the features separately. According to an aspect of an embodiment, the camera system is installed outside the purge volume of OPuS enclosure to mitigate the risks of open beam operation and avoid breaking purge. As used herein, the term “camera” is intended to encompass any device, system, or arrangement for capturing (converting) an image.

According to aspects of an embodiment, during alignment the landing position of the beam as revealed by a “florescence footprint” is positioned to be coincident with or overlap an alignment feature of a component being aligned. For example, where the optical component is a dichroic mirror within the OPuS, the alignment feature may be placed on the back support plate through the dichroic mirror to facilitate beam positioning.

Thus, according to some embodiments, use of the camera system minimizes the amount of open beam operation required during alignment. This significantly improves the safety of the alignment procedure. Also, the non-contact nature of this alignment system reduces the risks inherent in direct close optics handling including the risk of surface contamination. The system also makes possible a significant reduction in the field service time of the OPuS module due to elimination of the need to open the enclosure and break purge, and of manual target positioning.

According to an aspect of an embodiment there is disclosed an optical component comprising a sealed enclosure, the sealed enclosure including a window transparent to visible light, a first optical feature positioned at a first position within the enclosure, a second optical feature positioned at a second position within the enclosure, and an image integration module arranged to receive first optical feature light from the first optical feature and second optical feature light from the second optical feature and adapted to redirect the first optical feature light and the second optical feature light through the window to form an image from the first optical feature light collocated with an image from the second optical feature light. The optical component may be an optical pulse stretcher. The first optical feature may comprise a first mirror and the second optical feature may comprise a second mirror. The first mirror may comprise a first concave dichroic mirror and the second mirror may comprise a second concave dichroic mirror. The first optical feature and the second optical feature may be positioned substantially symmetrically with respect to the image integration module.

According to another aspect of an embodiment the image integration module may comprise a first reflective surface arranged to redirect light from the first optical feature and a second reflective surface arranged to redirect light from the second optical surface. The first reflective surface may comprise a first prism reflective surface of a first prism and the second reflective surface may comprise a second prism reflective surface of a second prism. The image integration module may comprise a prism having a first reflective surface oriented toward the first optical feature and a second reflective surface oriented toward the second optical feature. The image integration module may comprise two flat beveled mirrors.

According to another aspect of an embodiment the optical component may further comprise a third optical feature positioned at a third position within the enclosure and a fourth optical feature positioned at a fourth position within the enclosure, and the image integration module may be arranged to receive third optical feature light from the third optical feature and fourth optical feature light from the fourth optical feature and adapted to combine and redirect the third optical feature light and the fourth optical feature light through the window to form an image from the third optical feature light adjacent to an image from the fourth optical feature light.

According to another aspect of an embodiment the optical component may further comprise a camera system arranged to receive the first optical feature light and the second optical feature light through the window. The camera system may comprise a lens system arranged to receive the first optical feature light and the second optical feature light through the window and a camera arranged to receive the first optical feature light and the second optical feature light from the lens system. The optical component as may further comprise a folding mirror optically positioned between the image integration module and the window for turning an optical path of the first optical feature light and the second optical feature light.

According to another aspect of an embodiment at least one of the first optical feature and the second optical feature may be adjustable and may further comprise an actuator mechanically coupled to the at least one of the first optical feature and the second optical feature to adjust an orientation of the at least one of the first optical feature and the second optical feature.

According to another aspect of an embodiment the first optical feature may comprise a first fluorescent material and a first alignment feature and the second optical feature may comprise a second fluorescent material and a second alignment feature. The first optical feature may comprise a first mirror comprising a first substrate transparent to visible light and first reflective coating that is reflective to UV radiation and a first mirror support, and the second optical feature may comprise a second mirror comprising a second substrate transparent to visible light and second reflective coating that is reflective to UV radiation and a second mirror support. The first mirror support may comprise a first alignment feature on a front surface of the first mirror support and the second mirror support may comprise a second alignment feature on a front surface of the second mirror support. The first alignment feature may correspond to a position of an aligned beam footprint on the first mirror and the second alignment feature may correspond to a position of an aligned beam footprint on the second mirror.

According to another aspect of an embodiment the first optical feature may comprise a first reflective coating including the first fluorescent material and the second optical feature may comprise a second reflective coating including the second fluorescent material. The first fluorescent material may be provided on a back surface of the first substrate and the second fluorescent material may be provided on a back surface of the second substrate. The first fluorescent material may be provided on a front surface of the first mirror support and the second fluorescent material may be provided on a front surface of the second mirror support. The first mirror support may comprise the first fluorescent material and the second mirror support may comprise the second fluorescent material.

According to another aspect of an embodiment there is disclosed an optical component comprising a sealed enclosure, the sealed enclosure including a window transparent to visible light, a first optical feature positioned within a first field of view within the enclosure, a second optical feature positioned within the first field of view within the enclosure, and an image integration module arranged to receive first field of view light from the first field of view and adapted to combine and redirect the first field of view light through the window, and the image integration module may comprise a planar mirrored surface inclined with respect to a line passing through a center of the planar mirrored surface and substantially parallel to the first field of view by an angle θ given by the relationship

$\theta = {45 + {\left\lbrack {{{atan}\left\lbrack \frac{\frac{h}{2} - \frac{w}{s\sqrt{2}}}{s} \right\rbrack} - {{atan}\left( \frac{d}{s} \right)}} \right\rbrack/2.}}$

where h is a height of the first field of view, d is a vertical distance between the center of the first field of view to the center of the planar mirrored surface, and s is a horizontal distance between the first field of view and the center of the planar mirrored surface.

According to another aspect of an embodiment there is disclosed a method of aligning a plurality of optical features arranged in sealed enclosure having a window, the method comprising combining light from each of the optical features to produce a combined light signal, directing the combined light signal out of the enclosure through the window, and aligning at least some of the plurality of optical features based at least in part on the combined light signal. The method may further comprise imaging the combined light signal after directing the combined light signal out of the enclosure through the window using a camera system positioned outside of the sealed enclosure and arranged to receive the combined light signal. Each of the plurality of optical features may comprise an alignment feature and a fluorescent material arranged to generate a visible footprint of a UV beam impinging on the optical feature and the method may further comprise, before combining light from each of the optical features to produce a combined light signal, exposing each of the optical features to a beam of UV radiation, and generating an illuminated footprint of the beam of UV radiation on each of the optical features, and aligning at least some of the plurality of optical features based at least in part on the combined light signal may comprise aligning at least some of the plurality of optical features based at least in part on a positional relationship of the illuminated footprint and the alignment feature for each optical feature. The aligning may comprise adjusting of one or more of the plurality of optical features. Adjusting one or more of the plurality of optical features may comprise manually operating one or more actuators respectively mechanically coupled to the one or more of the plurality of optical features. Adjusting one or more of the plurality of optical features may comprise supplying a signal to actuate one or more motor driven actuators respectively mechanically coupled to the one or more of the plurality of optical features. Adjusting one or more of the plurality of optical features may comprise adjusting an orientation of one or more of the plurality of optical features.

According to another aspect of an embodiment there is disclosed a method of aligning a beam splitter in a confocal optical pulse stretcher, the confocal optical pulse stretcher comprising at least a first concave mirror and a second concave mirror confocal with the first concave mirror, the beam splitter being arranged optically (i.e., on an optical path between) between a first spot on the first concave mirror and a second spot on the second concave mirror, the beam splitter being further arranged to split a first incoming beam travelling in a first direction into a first beam portion directed to the first spot and a second beam portion continuing to travel in the first direction toward a retroreflector, the first beam portion traversing an optical path in the confocal optical pulse stretcher, the optical path including a segment between the second spot and the beam splitter, the first beam portion striking the second mirror at the second spot, the method comprising retroreflecting the second beam portion back to the beam splitter along a return path in a second direction opposed to the first direction, splitting the retroreflected second beam portion into a beam travelling from the beam splitter to the second spot and a third beam portion continuing to travel in the second direction, and aligning the split retroreflected second beam portion to be incident on the second mirror at the second spot. The method may further comprise causing a portion of the third beam to strike a fluorescent screen to create an image, and aligning the retroreflector based at least in part on whether the image has a predetermined characteristic. Causing at least a portion of the third beam to strike a fluorescent screen to create an image may comprise reflecting the at least a portion of the third beam using a reflective element with an aperture onto the fluorescent screen. Determining whether the image has a predetermined characteristic comprises determining whether the image includes light reflected symmetrically from a rim of the aperture.

According to another aspect of an embodiment there is disclosed a confocal optical pulse stretcher comprising a first concave mirror, a second concave mirror confocal with the first concave mirror, a retroreflector, and a beam splitter arranged optically between the first concave mirror and the second concave mirror, the beam splitter being further arranged to split a first beam travelling in a first direction into a first beam portion directed to the first concave mirror and a second beam portion travelling in the first direction, the first beam portion traversing a closed optical path in the confocal optical pulse stretcher, the optical path including a segment between the second concave mirror and the beam splitter, the retroreflector being arranged to retroreflect the second beam portion to produce a retroreflected beam travelling in a second direction opposed to the first direction, the beam splitter being further arranged to split the retroreflected beam into a third beam portion traversing the segment to be incident on the second concave mirror. The retroreflector may comprise a mirror with adjustable tip and tilt. The retroreflector may comprise a folding mirror with adjustable tip and tilt and a fixed mirror, the folding mirror being arranged to receive and redirect the second beam portion toward the fixed mirror and the fixed mirror being arranged to reflect the second beam portion back towards the folding mirror. The confocal optical pulse stretcher may further comprise a reflective element having an aperture arranged to receive retroflected light passing through the beam splitter and travelling in a second direction opposite the first direction and redirect the light and a fluorescent screen arranged to receive the redirected light. The reflective element may be oriented at an angle with respect to the light passing through the beam splitter and have an aperture. The UV reflective element may have a reflective surface arranged to redirect the light.

According to another aspect of an embodiment there is disclosed an apparatus for facilitating alignment of a beam splitter in a confocal optical pulse stretcher, the confocal optical pulse stretcher comprising at least a first concave mirror and a second concave mirror confocal with the first concave mirror, the confocal optical pulse stretcher additionally comprising a retroreflector, the beam splitter being arranged optically between the first concave mirror and the second concave mirror, the beam splitter being further arranged to split a first beam into a first beam portion directed to the first concave mirror and a second beam portion travelling in a first direction toward the retroreflector, the retroreflector being arranged to retroreflect the second beam portion to produce a retroreflected beam, the first beam portion traversing an optical path in the confocal optical pulse stretcher, the beam path including a segment between the second concave mirror and the beam splitter, the beam splitter being further arranged to split the retroreflected beam into a first portion travelling to the second mirror and a second portion passing through the beam splitter, the apparatus comprising a reflective element having an aperture arranged to receive the second portion of retroreflected beam light passing through the beam splitter and to redirect the light and a fluorescent screen arranged to receive the redirected light. The reflective element may be oriented at an angle with respect to the second portion of retroreflected beam passing through the beam splitter and have an aperture. The reflective element may have a reflective surface arranged to redirect the second portion of retroreflected beam.

Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. It is noted that the present invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the methods and systems of embodiments of the invention by way of example, and not by way of limitation. Together with the detailed description, the drawings further serve to explain the principles of and to enable a person skilled in the relevant art(s) to make and use the methods and systems presented herein. In the drawings, like reference numbers indicate identical or functionally similar elements.

FIG. 1 shows a schematic, not to scale, view of an overall broad conception of a photolithography system according to an aspect of the disclosed subject matter.

FIG. 2 shows a schematic, not to scale, view of an overall broad conception of a laser system used in a lithography system according to an aspect of the disclosed subject matter.

FIG. 3 is a diagram of an optical pulse stretcher according to an aspect of the disclosed subject matter.

FIG. 4 is a diagram showing various light paths within an optical pulse stretcher according to an aspect of the disclosed subject matter.

FIGS. 5A and 5B are diagrams showing various light paths within an optical pulse stretcher according to an aspect of the disclosed subject matter.

FIG. 6 is a partially perspective view of the arrangement of mirrors in one side of an optical pulse stretcher according to an aspect of the disclosed subject matter.

FIG. 7 is a diagram showing a conventional method of aligning optical features within an optical component.

FIGS. 8A and 8B are diagrams showing a system for aligning optical features according to an aspect of the disclosed subject matter.

FIG. 9A is a diagram showing a system for aligning optical features according to an aspect of the disclosed subject matter.

FIG. 9B is a diagram showing image collocation in a system for aligning optical features according to an aspect of the disclosed subject matter.

FIG. 10 is a diagram illustrating the conditions for achieving maximal field of view according to an aspect of the disclosed subject matter.

FIGS. 11A and 11B show alternative components for an image integration module according to an aspect of the disclosed subject matter.

FIG. 11C is a diagram showing the effects of placement of components within the optical image integration module on field of view according to an aspect of the disclosed subject matter.

FIG. 12A is a diagram showing a system for aligning optical features according to an aspect of the disclosed subject matter.

FIG. 12B is a diagram showing image collocation in a system for aligning optical features according to an aspect of the disclosed subject matter.

FIGS. 13A, 13B, 13C, and 13D show aspects of the structure of optical features according to aspects of the disclosed subject matter.

FIGS. 14A, 14B, 14C, and 14D show aspects of the structure of optical features according to aspects of the disclosed subject matter.

FIGS. 15A and 15B show the relative positioning of a UV footprint and an alignment mark before and after an alignment process, respectively, according to an aspect of the disclosed subject matter.

FIG. 16 is a flow chart illustrating a method of aligning optical features in optical component according to an aspect of the disclosed subject matter.

FIG. 17 is a diagram showing various light paths within an optical pulse stretcher according to an aspect of the disclosed subject matter.

FIG. 18 is a diagram showing various light paths within an optical pulse stretcher according to an aspect of the disclosed subject matter.

FIG. 19 is a flow chart illustrating a method of aligning optical features in optical component according to an aspect of the disclosed subject matter.

FIG. 20 is a flow chart illustrating a method of aligning optical features in an optical component according to an aspect of the disclosed subject matter.

FIG. 21 is a diagram showing various light paths within an optical pulse stretcher according to an aspect of the disclosed subject matter.

FIG. 22 is a diagram showing various light paths within an optical pulse stretcher according to an aspect of the disclosed subject matter.

FIG. 23A is a diagram showing various light paths within an optical pulse stretcher according to an aspect of the disclosed subject matter.

FIG. 23B is a diagram showing various alignment conditions of rays within an optical pulse stretcher according to an aspect of the disclosed subject matter.

FIG. 24 is a diagram showing various light paths within an optical pulse stretcher according to an aspect of the disclosed subject matter.

FIG. 25 is a diagram showing various light paths within an optical pulse stretcher according to an aspect of the disclosed subject matter.

FIG. 26 is a flow chart illustrating a method of aligning optical features in optical component according to an aspect of the disclosed subject matter.

FIG. 27 is a diagram showing various light paths within an optical pulse stretcher according to an aspect of the disclosed subject matter.

FIG. 28 is a diagram showing various light paths within an optical pulse stretcher according to an aspect of the disclosed subject matter.

FIGS. 29A-29C are diagrams showing some possible patterns resulting from various alignments of a retroreflected ray within an optical pulse stretcher according to an aspect of the disclosed subject matter.

FIGS. 30A-30C are diagrams depicting some aspects of an aperture plate within a module for aligning a retroreflected ray in an optical pulse stretcher according to an aspect of the disclosed subject matter.

Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art based on the teachings contained herein.

DETAILED DESCRIPTION

This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiment(s) merely exemplify the present invention. The scope of the present invention is not limited to the disclosed embodiment(s). The present invention is defined by the claims appended hereto.

The embodiment(s) described, and references in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, “an exemplary embodiment”, etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect and associate such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “on,” “upper,” “left,” “right” and the like, may be used herein for ease of description to describe one element's or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

Before describing specific embodiments in more detail, it is instructive to present an example environment in which embodiments of the present invention may be implemented. Referring to FIG. 1 , a photolithography system 100 includes an illumination system 105. As described more fully below, the illumination system 105 includes a light source that produces a pulsed light beam 110 and directs it to a photolithography exposure apparatus or scanner 115 that patterns microelectronic features on a wafer 120. The wafer 120 is placed on a wafer table 125 constructed to hold wafer 120 and connected to a positioner (not shown) configured to accurately position the wafer 120 in accordance with certain parameters.

The photolithography system 100 uses a light beam 110 having a wavelength in the deep ultraviolet (DUV) range, for example, with wavelengths of 248 nanometers (nm) or 193 nm. The minimum size of the microelectronic features that can be patterned on the wafer 120 depends on the wavelength of the light beam 110, with a lower wavelength permitting a smaller minimum feature size. When the wavelength of the light beam 110 is 248 nm or 193 nm, the minimum size of the microelectronic features can be, for example, 50 nm or less. The bandwidth of the light beam 110 can be the actual, instantaneous bandwidth of its optical spectrum (or emission spectrum), which contains information on how the optical energy of the light beam 110 is distributed over different wavelengths. The scanner 115 includes an optical arrangement having, for example, one or more condenser lenses, a mask, and an objective arrangement. The mask is movable along one or more directions, such as along an optical axis of the light beam 110 or in a plane that is perpendicular to the optical axis. The objective arrangement includes a projection lens and enables the image transfer to occur from the mask to the photoresist on the wafer 120. The illumination system 105 adjusts the range of angles for the light beam 110 impinging on the mask. The illumination system 105 also homogenizes (makes uniform) the intensity distribution of the light beam 110 across the mask.

The scanner 115 can include, among other features, a lithography controller 130, air conditioning devices, and power supplies for the various electrical components. The lithography controller 130 controls how layers are printed on the wafer 120. The lithography controller 130 includes a memory that stores information such as process recipes. A process program or recipe determines the length of the exposure on the wafer 120 based on, for example, the mask used as well as other factors that affect the exposure. During lithography, a plurality of pulses of the light beam 110 illuminate the same area of the wafer 120 to constitute an illumination dose.

The photolithography system 100 also preferably includes a control system 135. In general, the control system 135 includes one or more of digital electronic circuitry, computer hardware, firmware, and software. The control system 135 also includes memory which can be read-only memory and/or random access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including, by way of example, semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM disks.

The control system 135 can also include one or more input devices (such as a keyboard, touch screen, microphone, mouse, hand-held input device, etc.) and one or more output devices (such as a speaker or a monitor). The control system 135 may also include one or more programmable processors, and one or more computer program products tangibly embodied in a machine-readable storage device for execution by one or more programmable processors. The one or more programmable processors can each execute a program of instructions to perform desired functions by operating on input data and generating appropriate output. Generally, the processors receive instructions and data from the memory. Any of the foregoing may be supplemented by, or incorporated in, specially designed ASICs (application-specific integrated circuits). The control system 135 can be centralized or be partially or wholly distributed throughout the photolithography system 100.

Referring to FIG. 2 , an exemplary laser source system within the illumination system 105 is a pulsed laser source that produces a pulsed laser beam as the light beam 110. FIG. 2 shows illustratively and in block diagram a gas discharge laser system according to an embodiment of certain aspects of the disclosed subject matter. The gas discharge laser system may include, e.g., a solid state or gas discharge seed laser system 140, an amplification stage, e.g., a power ring amplifier (“PRA”) stage 145, relay optics 150 and laser system output subsystem 160. The seed system 140 may include, e.g., a master oscillator (“MO”) chamber 165.

The seed laser system 140 may also include a master oscillator output coupler (“MO OC”) 175, which may comprise a partially reflective mirror, forming with a reflective grating (not shown) in a line narrowing module (“LNM”) 170, an oscillator cavity in which the seed laser 140 oscillates to form the seed laser output pulse, i.e., forming a master oscillator (“MO”). The system may also include a line-center analysis module (“LAM”) 180. The LAM 180 may include an etalon spectrometer for fine wavelength measurement and a coarser resolution grating spectrometer. A MO wavefront engineering box (“WEB”) 185 may serve to redirect the output of the MO seed laser system 140 toward the amplification stage 145, and may include, e.g., beam expansion with, e.g., a multi prism beam expander (not shown) and coherence busting, e.g., in the form of an optical delay path (not shown).

The amplification stage 145 may include, e.g., a PRA lasing chamber 200, which may also be an oscillator, e.g., formed by seed beam injection and output coupling optics (not shown) that may be incorporated into a PRA WEB 210 and may be redirected back through the gain medium in the chamber 200 by a beam reverser 220. The PRA WEB 210 may incorporate a partially reflective input/output coupler (not shown) and a maximally reflective mirror for the nominal operating wavelength (e.g., at around 193 nm for an ArF system) and one or more prisms.

A bandwidth analysis module (“BAM”) 230 at the output of the amplification stage 145 may receive the output laser light beam of pulses from the amplification stage and pick off a portion of the light beam for metrology purposes, e.g., to measure the output bandwidth and pulse energy. The laser output light beam of pulses then passes through an optical pulse stretcher (“OPuS”) 240 and an output combined autoshutter metrology module (“CASMM”) 250, which may also be the location of a pulse energy meter. One purpose of the OPuS 240 may be, e.g., to convert a single output laser pulse into a pulse train. Secondary pulses created from the original single output pulse may be delayed with respect to each other. By distributing the original laser pulse energy into a train of secondary pulses, the effective pulse length of the laser can be expanded and at the same time the peak pulse intensity reduced. The OPuS 240 can thus receive the laser beam from the PRA WEB 210 via the BAM 230 and direct the output of the OPuS 240 to the CASMM 250. Other suitable arrangements may be used in other embodiments.

The PRA lasing chamber 200 and the MO 165 are configured as chambers in which electrical discharges between electrodes may cause lasing gas discharges in a lasing gas to create an inverted population of high energy molecules, including, e.g., Ar, Kr, and/or Xe, to produce relatively broad band radiation that may be line narrowed to a relatively very narrow bandwidth and center wavelength selected in a line narrowing module (“LNM”) 170, as is known in the art.

Typically, the tuning takes place in the LNM. A typical technique used for line narrowing and tuning of lasers is to provide a window at the back of the laser's discharge cavity through which a portion of the laser beam passes into the LNM. There, the portion of the beam is expanded with a prism beam expander and directed to a grating which reflects a narrow selected portion of the laser's broader spectrum back into the discharge chamber where it is amplified. The laser is typically tuned by changing the angle at which the beam illuminates the grating using an actuator such as, for example, a piezoelectric actuator.

In operation, the OPuS 240 stretches the excimer or other gas discharge laser, e.g., a molecular fluorine gas discharge laser, having a given pulse duration and TIS to a longer pulse having several peaks and a larger TIS.

FIG. 3 is a schematic diagram of front view of an example of an optical pulse stretcher 401 having first optical pulse stretcher 401 a and second optical pulse stretcher 401 b, according to some embodiments of the present disclosure. The optical pulse stretcher 401 receives input beam pulse 411 and stretches it to output a stretched output beam pulse 413.

According to some embodiments, and as discussed in more detail below, second optical pulse stretcher 401 b can include two or more (for example, three) stages of confocal optical pulse stretchers. In some examples, these three stages of confocal optical pulse stretcher can be positioned approximately parallel to each other in second optical pulse stretcher 401 b. In some embodiments, second optical pulse stretcher 401 b can be positioned perpendicular or approximately perpendicular to first optical pulse stretcher 401 a. In other words, in some embodiments, first optical pulse stretcher 401 a (e.g., an orthogonal optical pulse stretcher that may be positioned vertically) is positioned perpendicular or approximately perpendicular to the two or more (for example, three) stages of confocal optical pulse stretchers of second optical pulse stretcher 401 b which is positioned vertically in the figure. According to some embodiments, second optical pulse stretcher 401 b is designed such that it provides additional optical delay.

According to some embodiments, the extended optical pulse stretcher 401 combines two or more confocal optical pulse stretchers. For example, extended optical pulse stretcher 401 combines confocal optical pulse stretchers in the combination of 4 reflections, 4 reflections, 12 reflections, and 12 reflections per optical circuit configuration. According to some embodiments, the inclusion of the combination of different mirror separations and delay path lengths (e.g., 4 reflections and 12 reflections delay lengths) can result in very long pulse stretching and minimal efficiency losses.

According to some embodiments, second optical pulse stretcher 401 b can include three stages of confocal optical pulse stretchers. However, the embodiments of this disclosure are not limited to these examples, and second optical pulse stretcher 401 b can include other numbers of stages of confocal optical pulse stretchers. In some examples, the first stage of second optical pulse stretcher 401 b is discussed as having two mirrors. However, the embodiments of this disclosure are not limited to these examples and the first stage of second optical pulse stretcher 401 b can include other numbers (for example two or more) and/or configurations of mirrors. In some examples, the plurality of mirrors used in the first stage of second optical pulse stretcher 401 b are configured to generate four reflections of the laser beam between them.

In some examples, the second stage of second optical pulse stretcher 401 b is discussed as having four mirrors. However, the embodiments of this disclosure are not limited to these examples and the second stage of second optical pulse stretcher 401 b can include other numbers (for example four or more) and/or configurations of mirrors. In some examples, the plurality of mirrors used in the second stage of second optical pulse stretcher 401 b are configured to generate twelve reflections of the laser beam between them.

In some examples, the third stage of second optical pulse stretcher 401 b is discussed as having four mirrors. However, the embodiments of this disclosure are not limited to these examples and the third stage of second optical pulse stretcher 401 b can include other numbers (for example four or more) and/or configurations of mirrors. In some examples, the plurality of mirrors used in the third stage of second optical pulse stretcher 401 b are configured to generate twelve reflections of the laser beam between them.

According to some embodiments, first optical pulse stretcher 401 a and the stages of second optical pulse stretcher 401 b are designed such that optical delay increases from first optical pulse stretcher 401 a to second optical pulse stretcher 401 b. Also, the optical delay of each stage of second optical pulse stretcher 401 b increases from the first to the third stage. For example, first optical pulse stretcher 401 a (e.g., the orthogonal optical pulse stretcher) can have an optical delay. The first stage of second optical pulse stretcher 401 b can have a first optical delay equal to or greater than the optical delay of first optical pulse stretcher 401 a. The second stage of second optical pulse stretcher 401 b can have a second optical delay equal to or greater than the first optical delay. The third stage of second optical pulse stretcher 401 b can have a third optical delay equal to or greater than the second optical delay. According to some embodiments, the optical delay can be determined based on the distance that the beam travels within an optical pulse stretcher.

According to some embodiments, a first stage of second optical pulse stretcher 401 b can have an optical design including two mirrors (e.g., two lower mirrors of mirrors 501 and 502 in FIG. 3 ) that produce four reflections of the laser beam between them. Although this example is discussed with two mirrors, the first stage of second optical pulse stretcher 401 b can include other numbers of mirrors (for example, two or more mirrors). These mirrors can be positioned to generate four reflections of the laser beam between them. In some embodiments, the two mirrors of the first stage of second optical pulse stretcher 401 b can be separated from each other by a physical distance of about 2 m-4 m. For example, the physical distance can be about 2.5 m to 3.5 m. These distances are provided by way of example only and other distances can be used in other embodiments. In some examples, the first stage of second optical pulse stretcher 401 b can be capable of optical pulse stretching having, from example, an optical delay of about 60 ns-80 ns. For example, an optical delay of about 65 ns-75 ns. For example, an optical delay of about 70 ns-75 ns. It is noted that the example physical distance between the two mirrors and the example optical delays provided do not limit the embodiments of this disclosure. The first stage of second optical pulse stretcher 401 b can be designed such that various other physical distances and/or various optical delays are achieved.

According to some embodiments, mirrors (e.g., the two lower mirrors of mirrors 501 and 502) of the first stage of second optical pulse stretcher 401 b can include rectangular concave mirrors. For example, two large rectangular concave mirrors can be used but in other embodiments other shapes are used. According to some embodiments, the reflective surface of the mirrors can be spherically concave such that the distance between the two mirrors (e.g., the surfaces of the two lower mirrors of mirrors 501 and 502) of the first stage of second optical pulse stretcher 401 b is equal to (or about equal to) the radius of the curvature of each of the two mirrors. For example, the mirrors can be designed and positioned based on a telecentric design. The concave mirrors can be designed with orthogonal tip-tilt adjustment and also Z-axis (e.g., the direction of the propagation of beam) adjustment, according to some embodiments.

According to some embodiments, the first stage of second optical pulse stretcher 401 b can include additional optical elements. In one example, the first stage of second optical pulse stretcher 401 b can include a beam splitter used to split the laser beam and to generate copies of the laser beam. The beam splitter of the first stage of second optical pulse stretcher 401 b can have a reflectivity of, for example, about 45%-65%. In some examples, the beam splitter can have a reflectivity of about 50%-60%. But the embodiments of this disclosure are not limited to these examples and various other values of reflectivity can be used. In some examples, the reflectivity of the beam splitter can depend on and/or be calculated based on the reflectivity of the mirrors used in the first stage of second optical pulse stretcher 401 b.

According to some embodiments, a second stage of second optical pulse stretcher 401 b can have an optical design including four mirrors (e.g., four middle mirrors of mirrors 501 and 502 in FIG. 3 ) that produce 12 reflections of the laser beam between them. Although this example is discussed with four mirrors, the second stage of second optical pulse stretcher 401 b can include other numbers of mirrors (for example, four or more mirrors). These mirrors can be positioned to generate twelve reflections of the laser beam between them. In some embodiments, the two pairs of mirrors of the second stage of second optical pulse stretcher 401 b can be separated from each other by a physical distance of about 2 m-4 m. For example, the physical distance can be about 2.5 m to 3.5 m. These distances are provided by way of example only and other distances can be used in other embodiments. In some examples, the second stage of second optical pulse stretcher 401 b can be capable of optical pulse stretching having, from example, an optical delay of about 170 ns-210 ns. For example, an optical delay of about 180 ns-190 ns. For example, an optical delay of about 185 ns-195 ns. It is noted that the example physical distance between the two pairs of mirrors and the example optical delays provided do not limit the embodiments of this disclosure. The second stage of second optical pulse stretcher 401 b can be designed such that various other physical distances and/or various optical delays are achieved.

According to some embodiments, mirrors (e.g., the four middle mirrors of mirrors 501 and 502) of the second stage of second optical pulse stretcher 401 b can include rectangular concave mirrors. For example, four large rectangular concave mirrors can be used but in other embodiments other shapes are used. According to some embodiments, the reflective surface of the mirrors can be spherically concave such that the distance between the two pairs of mirrors (e.g., the surfaces of the two pairs of middle mirrors of mirrors 501 and 502) of the second stage of second optical pulse stretcher 401 b is equal to (or about equal to) the radius of the curvature of each of the four mirrors. For example, the mirrors can be designed and positioned based on a telecentric design. The concave mirrors can be designed with orthogonal tip-tilt adjustment, according to some embodiments.

According to some embodiments, the second stage of second optical pulse stretcher 401 b can include additional optical elements. In one example, the second stage of second optical pulse stretcher 401 b can include a beam splitter (middle beam splitter of beam splitters 503 of FIG. 3 ) used to split the laser beam and to generate copies of the laser beam. The beam splitter of the second stage of second optical pulse stretcher 401 b can have a reflectivity of, for example, about 45%-65%. In some examples, the beam splitter can have a reflectivity of about 50%-60%. But the embodiments of this disclosure are not limited to these examples and various other values of reflectivity can be used. In some examples, the reflectivity of the beam splitter can depend on and/or be calculated based on the reflectivity of the mirrors used in the second stage of second optical pulse stretcher 401 b.

Other details concerning optical pulse stretchers can be obtained from U.S. Pat. No. 7,369,597, titled “Laser Output Light Pulse Stretcher”, issued May 6, 2008, the entire contents of which are hereby incorporated by reference.

FIG. 4 illustrates a schematic view of part of the paths of laser beams in second optical pulse stretcher 401 b, according to some embodiments of the present disclosure.

As illustrated in FIG. 4 , laser beam 601, which is optically stretched using the stage of first optical pulse stretcher 401 a enters second optical pulse stretcher 401 b. Using first beam splitter 503 a, laser beam 601 is split into laser beam 603 and laser beam 605. Laser beam 605 enters the second stage of second optical pulse stretcher 401 b. Laser beam 603 enters the first stage of second optical pulse stretcher 401 b, which includes two mirrors. After four reflections from the two mirrors 501 a, 502 a of the first stage of second optical pulse stretcher 401 b as shown in FIG. 5A, part of the laser beam enters the second stage of second optical pulse stretcher 401 b by reflecting off beam splitter 503 a, the rest of beam will continue to further loops inside the optical pulse stretcher 400.

Laser beam 605 (and/or the laser beam from the first stage of second optical pulse stretcher 401 b) is split into laser beam 607 and laser beam 609. Laser beam 609 enters the third stage of second optical pulse stretcher 401 b. Laser beam 607 enters the second stage of second optical pulse stretcher 401 b, which includes four mirrors 501 c, 501 b, 502 b, and 501 c as shown in FIG. 5B. After twelve reflections from the four mirrors, identified by the numbers 1-12, of the second stage of second optical pulse stretcher 401 b part of the laser beam enters the third stage of enters second optical pulse stretcher 401 b by reflecting off beam splitter 503 b.

Laser beam 609 (and/or the laser beam from the second stage of the second optical pulse stretcher 401 b) is split into laser beam 611 and laser beam 613. Laser beam 613 is reflected using mirrors 505 a and 505 b back to first optical pulse stretcher 401 a. Laser beam 611 enters the third stage of second optical pulse stretcher 401 b, which includes four mirrors. After twelve reflections from the four mirrors of the third stage of second optical pulse stretcher 401 b, part of the laser beam is reflected toward first optical pulse stretcher 401 a using beam splitter 503 c and fold mirrors 505 a and 505 b (FIG. 4 ).

FIG. 6 illustrates a schematic view of part of the paths of laser beams in second optical pulse stretcher 401 b and parts of mirrors used in second optical pulse stretcher 401 b, according to some embodiments of the present disclosure.

In FIG. 6 , the five mirrors on one side of second optical pulse stretcher 401 b are illustrated. It will be understood that according to some embodiments an almost symmetric arrangement is also present in the second optical pulse stretcher 401 b. In this example, mirror 502 a of the first stage of second optical pulse stretcher 401 b is illustrated. A mirror (e.g., mirror 501 a) is on the other side of the first stage of second optical pulse stretcher 401 b, which is not illustrated in this view. In this example, one pair of mirrors 502 b and 502 c of the second stage of second optical pulse stretcher 401 b is illustrated. Another pair of mirrors (e.g., a pair of mirrors 501 b and 501 c) is on the other side of the second stage of second optical pulse stretcher 401 b, which is not illustrated in this view. Also, in this example, one pair of mirror 502 d and 502 e of the third stage of second optical pulse stretcher 401 b is illustrated. Another pair of mirrors (e.g., a pair of mirrors 501 d and 501 e) is on the other side of the third stage of second optical pulse stretcher 401 b, which is not illustrated in this view.

The following discussion is in terms of an arrangement in which the optical components within the OPuS such as mirrors are arranged in two banks which are almost left-right symmetric with respect to a central axis for the sake of having a concrete example to expedite explanation. It will be appreciated, however, that the principles elucidated herein may be applicable to other arrangements so that the specific examples described herein are not limiting. As sued herein, “almost symmetric” and “substantially symmetric” mean sufficiently symmetric that the OpuS can function for its intended purpose and an image integration module as described below can “see” all of the mirrors simultaneously. According to aspects of an embodiment, in such an arrangement an image integration module is arranged to collect object rays from both the left side optics and right side optics, which is this example are concave mirrors. Thus, in this arrangement, there are several right-left mirror pairs. A camera system including a camera and a lens system is positioned outside the sealed OPuS enclosure. The camera is arranged to collect rays through a sealing window which is transparent to light in the visible portion of the spectrum, that is, in the range of wavelengths from about 380 to about 700 nm. The rays produce an image with half of the image originating from left side concave mirror of a mirror pair and the other half of the image originating from the right side concave mirror of the mirror pair.

Against this backdrop, the conventional method of aligning the optical elements in an OPuS is described in connection with FIG. 7 . As seen in FIG. 7 , an OPuS 700 includes an enclosure 710. Within the enclosure 710 are positioned for optical elements 720, 730, 740, and 750. These optical elements may be, for example, mirrors. These optical elements must be aligned so that an incoming beam strikes the optical elements at the proper position. To perform this alignment procedure, the enclosure 710 is opened and an alignment card 760 is placed adjacent to the position of the optical surface of one of the optical elements, in the figure, optical element 750. The optical element is then aligned so that the beam falls on the proper position on the alignment card 760. As set forth above, this method entails several disadvantages such as the need to open the enclosure 710 and break purge and the need for an operator to insert their hands into the enclosure 710 in an open beam situation, which can expose the operator's hands to ultraviolet radiation. It also increases the risk of optical contamination and the resultant decrease in the useful lifetime of the optics.

According to an aspect of an embodiment, as shown in FIG. 8A, an OPuS 800 enabling an improved method of alignment includes enclosure 810. Located within the enclosure 810 are optical features 820, 830, 840, and 850. Located in a central portion of the enclosure 810 is an image integration module 860. As will be described in further detail below, this image integration module 860 collects light from the optical features and presents the light to a camera 890 through a sealing window 870 and a lens 880. In effect, the camera 890 “sees” all of the optical features within the enclosure 810 at the same time without any need to open the enclosure 810. This permits continuous observation of the alignment state of the OPuS 800 while avoiding the disadvantages of the prior methods. In some embodiments, the image integration module 860 is placed as closely to the camera system as possible to maximize the available field of view. Elements 835 and 855 are adjustors as will be explained in more detail below. FIG. 8B shows the placement of the image integration module 860 in relation to the arrangement shown in FIG. 5B. Again, the numbers 1-12 indicate the positions of the twelve reflections from the four mirrors.

FIG. 9A shows a possible implementation of the image integration module 860 according to an aspect of an embodiment. As shown, the image integration module 860 may be implemented as a pair of mirrored prisms 910, 920. The prism 910 is arranged receive light from a region of interest 825 including at least one optical feature 820 and redirect the light as shown out through the sealing window 870. Similarly, the prism 920 is arranged to receive light from a region of interest 835 and redirect the light as shown through the sealing window 870. Thus, a camera system including a camera and one or more lenses placed on the other side of the sealing window 870 simultaneously receives image-forming light from both the left side region of interest 825 and right side region of interest 835. In other words, the image integration module 860 collects object rays from both left side optical feature and the right side optical feature. A camera and lens system positioned outside the sealed OPuS enclosure 820 camera collects the object rays through the visible transparent sealing window and produces an image, half of the image being from left side optical feature and the other half being from right side optical feature.

FIG. 9B is a diagram showing the manner in which the image integration module 860 receives light from both the left side region of interest 825 containing at least one optical feature and right side region of interest 835 containing at least one optical feature and redirects the light to a position A from which a virtual object 825 a which is an image of the left side region of interest 825 and from which a virtual object 835 a which is an image of the right side region of interest 835 appear to be collocated, that is, positioned side-by-side, so that they can be viewed simultaneously by a single camera system placed at position A.

According to an aspect of an embodiment, the image integration module 860 may also be implemented using a pair of mirrored surfaces. As an example, a mirror 950, is shown in FIG. 10 , the other mirror being placed substantially symmetrically across axis 970. The conditions for a maximal field of view in such an arrangement are established geometrically also as shown in FIG. 10 . In the figure, h is the height of the field of interest 960, d is the vertical distance between the center of the field to the center of the mirror 950, and s is the horizontal distance between the field and the mirror 950. The upper part of the figure is the position of a virtual image viewed from below (in the figure) mirror. The angle θs for a maximal field of view between a line 980 parallel to the field of interest 960 and passing through a center of the mirror 950 is then given by the relationships:

${\theta 1} = \left( \frac{d}{s} \right)$ ${\theta 2} = \left\lbrack \frac{\frac{h}{2} - \frac{w}{2\sqrt{2}}}{s} \right\rbrack$ ${\theta 3} = \frac{90 - {\theta 1} - {\theta 2}}{2}$

so that the angle θs, which is the angle of inclination of the mirror 950 with respect to the optical vertical is given by

${\theta s} = {{{\theta 2} + {\theta 3}} = {\frac{{90} - {\theta 1} + {\theta 2}}{2} = {45 + {\left\lbrack {{{atan}\left\lbrack \frac{\frac{h}{2} - \frac{w}{2\sqrt{2}}}{s} \right\rbrack} - {{atan}\left( \frac{d}{s} \right)}} \right\rbrack/2.}}}}$

As mentioned, the image integration module 860 may be implemented in any one of a number of ways. According to aspects of some embodiments, the image integration module 860 may be implemented as a pair of prisms 910, 920 as shown in FIG. 9 , or as a single prism 1000 with two mirrored surfaces 1010 and 1020 as shown in FIG. 11A, or as two flat beveled mirrors 1030 and 1040 as shown in FIG. 11B. However, the image integration module 860 is implemented, in some embodiments the gap between the optical elements or mirrored surfaces is minimized so as not to waste the portion of the field of view lost to gaps as depicted in FIG. 11C. In FIG. 11C the arrow A depicts the field of view from one of the optical features, e.g., mirror and the arrow B depicts the field of view from the other, paired optical feature, and the arrow C depicts the field of view lost due to the gap between the prisms 910, 920.

FIG. 9A depicts an arrangement in which the light passing through the sealing window 870 travels directly in a straight line to the lens system 880. For some embodiments it may be advantageous to interpose additional optical elements in the path from the sealing window 870 to the lens system 880. For example, FIG. 12A shows an arrangement in which a folding mirror 1100 is placed in this path to fold the optical path and create the possibility of providing more compact arrangements. In the arrangement in FIG. 12A the mirror 1100 is positioned within the enclosure to obtain a larger field of view. This arrangement also provides the ability to adjust the image orientation and improve matching of the size and shape of the field and the image sensor in the camera. FIG. 12B shows an arrangement in which the prisms 910 a, 920 a are partially rotated to fold the optical path. FIG. 12B shows the manner in which the image integration module 860 a receives light from both the left side region of interest 825 and right side region of interest 835 and redirects the light to a position B from which a virtual object 825 b which is an image of the left side region of interest 825 and from which a virtual object 835 b which is an image of the right side region of interest 835 appear to be collocated, that is, positioned side-by-side, so that they can be viewed simultaneously by a single camera system placed at position B. As shown, this arrangement also provides the ability to adjust the image orientation and improve matching of the size and shape of the field and the image sensor in the camera.

For some embodiments it may also be advantageous to enhance the visibility of the region of interest by providing an alignment feature and by using visible (to the camera) light from fluorescence produced by absorption of ultraviolet light. Using the example of a dichroic mirror as the optical element to be aligned, the mirror in general is supported by a mirror supporting plate 1300 which includes a support 1310 and at least one alignment feature 1320. According to some embodiments, as shown in FIG. 13 , a dichroic mirror assembly 1330 includes a substrate 1340 which transmits visible light as shown in FIG. 13B and a UV reflective coating 1360 (FIG. 13D). The dichroic mirror assembly 1330 is overlaid on top of the mirror supporting plate 1300 to produce the sandwich-like structure shown in FIGS. 13C and 13D. UV radiation striking the dichroic mirror assembly 1330 will create a visible beam fluorescence footprint 1350 in a manner described more fully below. The fluorescence generated by the ultraviolet radiation and the back alignment feature can be observed by the camera operating in, for example, the visible spectrum range. By comparing the location of the fluorescence and the alignment feature through the mirror or substrate the system can easily be aligned as described below. While visible light is used in this example, it will be understood that radiation outside of the visible portion of the spectrum may be used.

According to aspects of various embodiments, the illuminated beam footprint may be generated on exposure to UV in any one of a number of different ways. For example, as shown in FIG. 14A, the UV reflective coating 1360 may be selected to be one that exhibits intrinsic fluorescence properties upon exposure to UV. This is indicated in FIGS. 14A-14D with the thick arrow indicating incident UV radiation and the wavy arrows indicating light produced by fluorescence. Alternatively, as shown in FIG. 14B, a back surface of the substrate 1340 may be provided with a fluorescent coating 1370 with light being generated by leakage of UV through the reflective coating 1360. The term “back surface” when referring to the substrate 1340 means the substrate surface that faces away from the incoming UV radiation. Alternatively, as shown in FIG. 14C, a front surface of the support 1310 may be provided with a fluorescent coating 1380 with light being generated by leakage of UV through the reflective coating 1360. The term “front surface” when referring to the support 1310 means the support surface that faces in the direction of incoming UV radiation. Alternatively, as shown in FIG. 14D, the support 1310 may be made of a fluorescent material with light being generated by leakage of UV through the reflective coating 1360.

FIG. 15A is a plan view of dichroic mirror assembly 1330 overlaid on top of the mirror supporting plate 1300 with alignment feature 1320. The mirror supporting plate 1300 with alignment feature 1320 are shown in phantom as they are behind the dichroic mirror assembly 1330. UV radiation striking the dichroic mirror assembly 1330 creates a visible beam fluorescence footprint 1350 as described. FIG. 15A shows an unaligned position in which the fluorescence footprint 1350 does not sufficiently coincide with the alignment feature 1320. FIG. 15B shows an aligned position in which the fluorescence footprint 1350 does sufficiently coincide with the alignment feature 1320. This is accomplished by alignment so that the UV beam lands on the dichroic mirror assembly 1330 in correct locations (two in the example) which due to fluorescence light up and reveal the relative positioning of the UV footprint and the alignment feature. To correct alignment, the orientation of one or more mirrors is adjusted so that the beam lands in the correct positions on all of the mirrors. The image of the beam landing juxtaposed with the alignment feature is captured by the camera, that is, converted to a digital image which an operator can view while performing an alignment operation.

According to some embodiments, the alignment of the pulse stretcher requires that at least some of the mirrors be adjustable, e.g., in the case of a four mirror arrangement, at least two of the four imaging relay mirrors be adjustable. Each of the two adjustable mirrors has tip/tilt adjustment creating a total of four degrees of freedom. The two adjustable mirrors may be located at opposite ends of the OPuS because of the confocal design of the system. The adjustable mirrors can also be designed with Z-axis (e.g., the direction of the propagation of beam) adjustment, according to some embodiments.

Typically, adjustments on these components to carry out alignment are made using a through-the-wall adjustor (“TWA”) such as adjustors 855 and 835 (FIG. 8A). These involve the use of a hand-manipulated hex driver to tip or tilt or translate an optic or module. TWAs may provide a sealed mechanical feed through to certain adjustments, e.g., accessed through the covers via a sealed mechanical feed-through. Adjustment can also be carried out with an electrically actuated TWA instead of a manually actuated TWA. A motor is mechanically coupled to the TWA. For example, the motor may have a shaft to which a hex adaptor is attached so that when the motor turns the shaft, the hex driver also turns, causing the end of the TWA to translate along its axis according to the direction of rotation of the shaft. Use of an electrically actuated TWA enables automation of the alignment process with the digital images from the camera 890 being conveyed to the control system 135 (FIG. 1 ) which in turn analyzes the images and actuates the TWAs to carry out alignment.

It should be understood that alignment may entail adjusting only one optical feature and that causing the beam to impinge on the proper part of a first optical feature may require adjusting a second optical feature optical feature

FIG. 16 is a flow chart showing a process for aligning optical features positioned within a sealed enclosure in accordance with aspects of embodiments. In a step S10 the optical features within the enclosure are exposed to a beam of UV radiation. The beam of UV radiation makes the footprint of the UV radiation beam visible. In a step S20 the light generated by the UV radiation beam fluorescence is combined into a single image from the optical features. The single image is conveyed outside of the enclosed chamber to a camera in a step S30. In a step S40 the features are aligned based on the image made from the light combined from the features, either by a technician while viewing the image captured by the camera or by a control system as described above. In essence, in instances where each of the features includes an alignment feature, the alignment is determined based on the positional relationship within the image of the beam footprint and the alignment feature for each optical feature.

As shown in FIG. 17 , a confocal 4× OpuS 1710 may include a beam splitter 1720 for coupling light into and out of the confocal OpuS 1710. The confocal OpuS 1710 also includes confocal concave mirrors CM1 and CM2 each having a focal length f and being separated by a distance d which is approximately equal to twice the focal length f. Alignment of the beam splitter 1720 is challenging due to the number of steps involved in the alignment and its dependence on other mirror alignment.

In the example of a 4× OpuS 1710 as shown in FIG. 17 , the position of the first beam landing spot on CM1, SP1, is controlled by the orientation of the beam splitter 1720. The position of the first beam landing spot on CM2, SP2, is controlled by CM1 orientation, which typically is provided with adjustable tip and tilt, and is also influenced by orientation of the beam splitter 1720. The position of the second beam landing spot on CM1, SP3, is controlled by CM2 orientation, orientation of the beam splitter 1720, and CM1 orientation. Here and elsewhere, the term “orientation” refers to at least tip and tilt orientation. The position of the second beam landing spot on CM2, SP4, however, is controlled almost solely by orientation of the beam splitter 1720. According to an aspect of an embodiment, the beam splitter 1720 is placed laterally asymmetrically with the distance between SP4 and the beam splitter 1720 being about six times the distance between the beam splitter 1720 and SP1.

As shown in FIG. 18 , a confocal 12× OpuS 1810 may include abeam splitter 1820 for coupling light into and out of the confocal OpuS 1810. The confocal OpuS 1810 also includes first confocal concave mirror pair CM1 and CM2 each having a focal length f and being separated by a distance d which is approximately equal to twice the focal length f. The confocal OpuS 1810 also includes second confocal concave mirror pair CM3 and CM4 each having a focal length f and being separated by a distance d which is approximately equal to twice the focal length f.

In the example of a 12× OpuS 1810 as shown in FIG. 18 , the position of the first beam landing spot on CM1, SP1, is controlled by the orientation of the beam splitter 1820. The position of the first beam landing spot on CM4, SP2, is controlled by CM1 orientation, which typically is provided with adjustable tip and tilt, and is also influenced by orientation of the beam splitter 1820. The position of the second beam landing spot on CM1, SP3, is controlled by CM4 orientation, orientation of the beam splitter 1820, and CM1 orientation. The position of the first beam landing spot on CM4, SP4, however, is controlled almost exclusively by orientation of the beam splitter 1820.

These pulse stretchers may be arrayed serially in stages. For example, the design of an overall OpuS may include one 4× OpuS alignment block as shown in FIG. 17 and two 12× OpuS alignment blocks as shown in FIG. 18 .

Currently alignment of the beam splitter in, for example, a 4× OpuS alignment block, involves several alignment steps that have to be performed so the beam can reach SP4 to enable beam splitter adjustment to align SP1 and SP4 together. If the beam falls off one of the concave mirrors during beam splitter alignment, additional realignment steps are required

FIG. 19 is a flow chart describing a method for aligning a beam splitter in a 4× OpuS alignment block without benefit of the subject matter disclosed herein. In a step S100 the beam splitter is aligned so that the beam landing spot SP1 is aligned to the proper beam location on the first concave mirror CM1. In a step S110, the concave mirror CM1 is aligned so that the landing spot SP2 is aligned to the proper beam location on the second concave mirror CM2. In a step S120, CM2 is aligned so that the third landing spot SP3 is aligned to the proper beam location on the first concave mirror CM1. In a step S130 the beam splitter is again aligned so that SP1 and SP4 are aligned to the proper beam location. In a step S140 it is determined whether the beam fell off of CM1 or CM2 during the alignment process. If the beam did fall off one of the concave mirrors during the alignment process, then the process reverts to step S110 and must be re-performed. If the beam did not fall off one of the concave mirrors, then the process ends in step S150.

FIG. 20 is a flow chart describing a method for aligning abeam splitter in a 12× OpuS alignment block without benefit of the subject matter disclosed herein. In a step S200 the beam splitter is aligned so that the beam landing spot SP1 is aligned to the proper beam location on the first concave mirror CM1. In a step S210, the concave mirror CM1 is aligned so that the landing spot SP2 is aligned to the proper beam location on the concave mirror CM4. In a step S220, CM4 is aligned so that the third landing spot SP3 is aligned to the proper beam location on the first concave mirror CM1. In a step S230 the beam splitter is again aligned so that SP1 and SP4 are aligned to the proper beam location. In a step S240 it is determined whether the beam fell off of CM1 or CM4 during the alignment process. If the beam did fall off one of these concave mirrors during the alignment process, then the process reverts to step S210 and must be re-performed. If the beam did not fall off one of the concave mirrors, then the process ends in step S250.

In an arrangement such as that described above in which an OpuS is made up of multiple blocks, the overall alignment process includes many steps, and potentially even more steps if the beam “falls off of,” i.e., is aimed so that it fails to intercept, a mirror. It thus may take an extended period of time to perform an overall alignment procedure. This undesirably results in a longer MTTR (mean time to repair) (about 30 min to 60 min) during laser installation and service. Reducing MTTR is a key to improving system availability and reducing service cost.

One way to reduce MTTR exploits the fact that in a well-aligned confocal OpuS there is an essentially perfect collinear relation of the direct pass beam and the split beam after it has traversed the OpuS. Referring to FIG. 21 , the incoming ray 2100, that is, the ray travelling upward in the figure, entering the block 2110, and hitting the beam splitter 2120, results in an “outgoing ray” 2130, that is, the direct pass ray 2130 travelling upward in FIG. 21 . The other portion of the incoming ray 2100 is deflected to SP1 as ray 2150 by the beam splitter 2120. This ray 2150 traverses a closed path in the OpuS 2110, one segment of which is ray 2140 travelling from SP4 to the beam splitter 2120 which is collinear with ray 2150. The ray 2140 is defected upward by the bean splitter 2120 as ray 2160 which in a well-aligned OpuS will coincide with beam 2130. This is also shown in FIG. 22 and FIG. 23 .

Because ray 2130 is collinear with ray 2160, if ray 2130 is retroreflected, the portion of the ray 2130 split off by the beam splitter 2120 and directed to SP4, ray 2170 in FIG. 23A, will be collinear with ray 2140 and both beams will overlap on SP4. Thus, SP4 can be re-established with a simple retro-reflected beam of ray 2130 without alignment of CM1 and CM2. This creates the possibility of greatly simplifying the beam splitter alignment procedure.

This is also shown in FIG. 23A. There, the incoming ray 2100 is split by beam splitter 1720 with a portion of the ray 2130 hitting a retroreflector 2200. As is known, a retroreflector (sometimes called a retroflector or cataphote) is a device or surface that reflects radiation back to its source. The retroreflector 2200 reflects ray 2130 into ray 2135 which strikes beam splitter 2100 and is reflected toward spot SP4 as ray 2170. At the same time, ray 2140 from SP4 to the beam splitter 1720 traverses the same path between SP4 and beam splitter 1720 thus making rays 2140 and 2170 collinear. This means that the footprints of the rays meet and overlap at SP4. A diagram of this overlap condition is shown in FIG. 23B. There, the portion of concave mirror CM2 having spot SP4 is struck by rays 2170 and 2140. As indicated above, the portion of CM2 where spot SP4 occurs may be provided with a material that fluoresces under ultraviolet light thereby making the beam landing spots visible. Note that while the landing spots for ray 2140 and ray 2170 will in general be mapped precisely to each other, the landing spots may not be perfectly circular as shown because, for example, ray 2140 may be clipped by CM1 or CM2 if ray 2140 falls off CM1 and CM2. The landing spots for both beams will, however, in general move together in sync when the beam splitter 1720 is adjusted. Circle 2180, shown in a dashed line, indicates a preferred beam location to which rays 2140 and 2170 are steered in unison by adjusting the beam splitter 1720 (FIG. 23A).

FIG. 24 shows this arrangement for a 12× OpuS block 2300. There, the incoming ray 2310 is split by beam splitter 2320 with a portion of the ray 2130 passing through the beam splitter 2320, ray 2330, hitting retroreflector 2340. The retroreflector 2340 reflects ray 2330 into ray 2350 which strikes beam splitter 2320 and is reflected toward spot SP4 as ray 2360. At the same time, for a well aligned 12X OPuS, there is a perfect collinear relation of the direct pass beam 2360 and beam 2370 through the OpuS. In other words, ray 2370 from SP4 to the beam splitter 2320 traverses the same path between SP4 and beam splitter 2320 thus making rays 2360 and 2370 collinear. This means that the rays meet and overlap essentially perfectly at SP4.

The above assumes the beam splitter 2120 has no thickness but the above conclusion holds even when the beam splitter 2120 has a nonzero thickness. The above conclusion also holds true for 12× OpuS alignment because SP4 can be easily replicated with the retro-reflected beam. With SP1 and SP4 visible, for example by the camera method described above, the beam splitter can be adjusted based solely on these two beam positions. This provides for a greatly simplified alignment procedure for the OpuS beam splitter which can be performed significantly more quickly.

The above described arrangements offer particular advantages in multi-stage optical pulse stretchers as shown in FIG. 25 . In FIG. 25 , a first stage optical pulse stretcher 2500 includes two confocal mirrors 2510 and 2520 and receives an input ray 2530 which is split by beam splitter 2540 into a beam that travels towards SP1 and the other part of which is split to travel upward in the figure to a second beep splitter 2550 in a second stage 2560 of the optical pulse stretcher. This stage 2560 also has a first mirror 2570 the second mirror 2580 and so on as indicated by the dots until an nth stage 2590 of the optical pulse stretcher with its beam splitter 2600 and mirrors 2610 and 2620. Above this nth stage is a retro reflector 2630 which reflects the beam traveling upward in the figure. The reflected ray strikes these beam splitters and is successively split with each offshoot ray creating a spot corresponding to SP4. Thus, all stages can be aligned using a single input ray and a single retroreflector.

For example, FIG. 26 shows a simplified procedure for aligning abeam splitter in a multi-stage OPuS commencing with a step S300 in which the retroreflector is aligned to reflect the “ray out” beam, that is, the ray that passes through the beam splitter. Then, in a step S310 the beam splitter is aligned based on the position of SP1 and the position of SP4 in the first stage OpuS. Then, in a step S320, the beam splitter in the second stage of the OPuS is aligned based on spot SP1 and spot SP4 in the stage two OPuS and so forth until step S330 pertaining to the final stage of the OpuS in which the beam splitter N is aligned based on the position of SP1 and SP4 in the stage N OPuS, at which point the procedure concludes with a step S340.

FIG. 27 shows an arrangement similar to that of FIG. 25 except that the retroreflector is implemented as a turning mirror or folding mirror 2700 and reflector 2710. This arrangement permits folding of the optical path and possibly a more compact arrangement for the retroreflector.

Proper adjustment of the retroreflector can be achieved by looking at the position of the retroreflected beam after passing back out of the first stage of the OpuS. An arrangement to achieve this is shown in FIG. 28 in which the retroreflected beam 2800 enters a module 2810 including a plate 2820 having an aperture 2830. If the retroreflector is properly adjusted, the ray 2800 will illuminate the edges of the aperture 2830. The plate 2820 is reflective with respect to ultraviolet radiation either by virtue of the material of which it was made or, as shown, by having a reflective coating 2840. The reflections from the rim of aperture 2830 shine on a fluorescent screen 2850. The alignment of the retroreflector can be assessed depending on the characteristics of the image on the fluorescent screen 2850.

FIGS. 29A, 29B, and 29C show various possibilities for the image on the fluorescent screen. FIG. 29A shows a cross-section of the aperture image in which the profile tracks the edge of the aperture symmetrically indicating that the retroreflector is properly adjusted. Note that the image of the circular aperture is somewhat elliptical because the plate is at an angle with respect to the retroreflected beam. FIG. 29B shows a cross-section of the aperture image in which the profile is off-center with respect to the aperture indicating that the retroreflector is not properly oriented. FIG. 29C is another example of a cross-section of the aperture image in which the profile does not track the edge of the aperture symmetrically indicating that the retroreflector is not properly adjusted.

FIGS. 30A, 30B, and 30C show various details of an aperture plate to be used in accordance with certain aspects of an embodiment. FIG. 30A is a side cutaway view of the aperture plate 2820 with aperture 2830 arranged to receive the retroreflected beam 2800 and reflecting a portion of the beam 2860 ideally from all of the edges of the aperture 2830. As noted, a reflective coating 2840 may be placed on the side of the plate 2820 on which the beam 2800 is incident. The aperture has an angled profile to avoid obstruction of a beam passing through the aperture when the plate is tilted. These details may also be seen in FIGS. 30B and 30C.

The diameter to the retroreflected beam travelling downward in the figure will be slighter larger than the aperture due to the finite divergence of laser beam, and the reflected beam will exhibit similar profile as the aperture. The entire module 2810 (FIG. 28 ) can be inserted into the beam during alignment of the retroreflected beam and can be removed from beam once alignment is finished. The plate 2820 may be made of a material such as aluminum which has a high intrinsic UV reflectivity when polished, for example, by diamond turning. The plate 2820 can also be built out glass/ceramic material with surface coating with a UV reflective coating.

The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.

The foregoing description of the specific embodiments will so fully reveal the general nature of the present invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance.

The embodiments can be further described using the following clauses:

-   -   1. A confocal optical pulse stretcher comprising:     -   a first concave mirror;     -   a second concave mirror confocal with the first concave mirror;     -   a retroreflector; and     -   a beam splitter arranged optically between the first concave         mirror and the second concave mirror, the beam splitter being         further arranged to split a first beam travelling in a first         direction into a first beam portion directed to the first         concave mirror and a second beam portion travelling in the first         direction, the first beam portion traversing a closed optical         path in the confocal optical pulse stretcher, the optical path         including a segment between the second concave mirror and the         beam splitter, the retroreflector being arranged to retroreflect         the second beam portion to produce a retroreflected beam         travelling in a second direction opposed to the first direction,         the beam splitter being further arranged to split the         retroreflected beam into a third beam portion traversing the         segment to be incident on the second concave mirror.     -   2. A confocal optical pulse stretcher of clause 1 wherein the         retroreflector comprises a mirror with adjustable tip and tilt.     -   3. A confocal optical pulse stretcher of clause 1 wherein the         retroreflector comprises a folding mirror with adjustable tip         and tilt and a fixed mirror, the folding mirror being arranged         to receive and redirect the second beam portion toward the fixed         mirror and the fixed mirror being arranged to reflect the second         beam portion back towards the folding mirror.     -   4. A confocal optical pulse stretcher of clause 1 further         comprising a reflective element having an aperture arranged to         receive retroflected light passing through the beam splitter and         travelling in a second direction opposite the first direction         and redirect the light; and a fluorescent screen arranged to         receive the redirected light.     -   5. A confocal optical pulse stretcher of clause 4 wherein the         reflective element is oriented at an angle with respect to the         light passing through the beam splitter and has an aperture.     -   6. A confocal optical pulse stretcher of clause 4 wherein the UV         reflective element has a reflective surface arranged to redirect         the light.     -   7. Apparatus for facilitating alignment of a beam splitter in a         confocal optical pulse stretcher, the confocal optical pulse         stretcher comprising at least a first concave mirror and a         second concave mirror confocal with the first concave mirror,         the confocal optical pulse stretcher additionally comprising a         retroreflector, the beam splitter being arranged optically         between the first concave mirror and the second concave mirror,         the beam splitter being further arranged to split a first beam         into a first beam portion directed to the first concave mirror         and a second beam portion travelling in a first direction toward         the retroreflector, the retroreflector being arranged to         retroreflect the second beam portion to produce a retroreflected         beam,     -   the first beam portion traversing an optical path in the         confocal optical pulse stretcher, the beam path including a         segment between the second concave mirror and the beam splitter,     -   the beam splitter being further arranged to split the         retroreflected beam into a first portion travelling to the         second mirror and a second portion passing through the beam         splitter, the apparatus comprising:     -   a reflective element having an aperture arranged to receive the         second portion of retroreflected beam light passing through the         beam splitter and to redirect the light; and     -   a fluorescent screen arranged to receive the redirected light.     -   8. Apparatus of clause 7 wherein the reflective element is         oriented at an angle with respect to the second portion of         retroreflected beam passing through the beam splitter and has an         aperture.     -   9. Apparatus of clause 7 wherein the reflective element has a         reflective surface arranged to redirect the second portion of         retroreflected beam.     -   10. A method of aligning a beam splitter in a confocal optical         pulse stretcher,     -   the confocal optical pulse stretcher comprising at least a first         concave mirror and a second concave mirror confocal with the         first concave mirror,     -   the beam splitter being arranged optically between a first spot         on the first concave mirror and a second spot on the second         concave mirror, the beam splitter being further arranged to         split a first incoming beam travelling in a first direction into         a first beam portion directed to the first spot and a second         beam portion continuing to travel in the first direction toward         a retroreflector,     -   the first beam portion traversing an optical path in the         confocal optical pulse stretcher, the optical path including a         segment between the second spot and the beam splitter, the first         beam portion striking the second mirror at the second spot,     -   the method comprising:     -   retroreflecting the second beam portion back to the beam         splitter along a return path in a second direction opposed to         the first direction;     -   splitting the retroreflected second beam portion into a beam         travelling from the beam splitter to the second spot and a third         beam portion continuing to travel in the second direction; and     -   aligning the split retroreflected second beam portion to be         incident on the second mirror at the second spot.     -   11. The method of clause 10 further comprising causing a portion         of the third beam portion to strike a fluorescent screen to         create an image, and aligning the retroreflector based at least         in part on determining whether the image has a predetermined         characteristic.     -   12. The method of clause 11 wherein causing at least a portion         of the third beam portion to strike a fluorescent screen to         create an image comprises reflecting the at least a portion of         the third beam portion using a reflective element with an         aperture onto the fluorescent screen.     -   13. The method of clause 11 wherein determining whether the         image has a predetermined characteristic comprises determining         whether the image includes light reflected symmetrically from a         rim of the aperture.

The preceding and other implementations are within the scope of the following claims. 

1. A confocal optical pulse stretcher comprising: a first concave mirror; a second concave mirror confocal with the first concave mirror; a retroreflector; and a beam splitter arranged optically between the first concave mirror and the second concave mirror, the beam splitter being further arranged to split a first beam travelling in a first direction into a first beam portion directed to the first concave mirror and a second beam portion travelling in the first direction, the first beam portion traversing a closed optical path in the confocal optical pulse stretcher, the optical path including a segment between the second concave mirror and the beam splitter, the retroreflector being arranged to retroreflect the second beam portion to produce a retroreflected beam travelling in a second direction opposed to the first direction, the beam splitter being further arranged to split the retroreflected beam into a third beam portion traversing the segment to be incident on the second concave mirror.
 2. The confocal optical pulse stretcher as in claim 1 wherein the retroreflector comprises a mirror with adjustable tip and tilt.
 3. The confocal optical pulse stretcher as in claim 1 wherein the retroreflector comprises a folding mirror with adjustable tip and tilt and a fixed mirror, the folding mirror being arranged to receive and redirect the second beam portion toward the fixed mirror and the fixed mirror being arranged to reflect the second beam portion back towards the folding mirror.
 4. The confocal optical pulse stretcher as in claim 1 further comprising a reflective element having an aperture arranged to receive retroflected light passing through the beam splitter and travelling in a second direction opposite the first direction and redirect the light; and a fluorescent screen arranged to receive the redirected light.
 5. The confocal optical pulse stretcher as in claim 4 wherein the reflective element is oriented at an angle with respect to the light passing through the beam splitter and has an aperture.
 6. The confocal optical pulse stretcher as in claim 4 wherein the UV reflective element has a reflective surface arranged to redirect the light.
 7. Apparatus for facilitating alignment of a beam splitter in a confocal optical pulse stretcher, the confocal optical pulse stretcher comprising at least a first concave mirror and a second concave mirror confocal with the first concave mirror, the confocal optical pulse stretcher additionally comprising a retroreflector, the beam splitter being arranged optically between the first concave mirror and the second concave mirror, the beam splitter being further arranged to split a first beam into a first beam portion directed to the first concave mirror and a second beam portion travelling in a first direction toward the retroreflector, the retroreflector being arranged to retroreflect the second beam portion to produce a retroreflected beam, the first beam portion traversing an optical path in the confocal optical pulse stretcher, the beam path including a segment between the second concave mirror and the beam splitter, the beam splitter being further arranged to split the retroreflected beam into a first portion travelling to the second mirror and a second portion passing through the beam splitter, the apparatus comprising: a reflective element having an aperture arranged to receive the second portion of retroreflected beam light passing through the beam splitter and to redirect the light; and a fluorescent screen arranged to receive the redirected light.
 8. The apparatus as in claim 7 wherein the reflective element is oriented at an angle with respect to the second portion of retroreflected beam passing through the beam splitter and has an aperture.
 9. The apparatus as in claim 7 wherein the reflective element has a reflective surface arranged to redirect the second portion of retroreflected beam.
 10. A method of aligning a beam splitter in a confocal optical pulse stretcher, the confocal optical pulse stretcher comprising at least a first concave mirror and a second concave mirror confocal with the first concave mirror, the beam splitter being arranged optically between a first spot on the first concave mirror and a second spot on the second concave mirror, the beam splitter being further arranged to split a first incoming beam travelling in a first direction into a first beam portion directed to the first spot and a second beam portion continuing to travel in the first direction toward a retroreflector, the first beam portion traversing an optical path in the confocal optical pulse stretcher, the optical path including a segment between the second spot and the beam splitter, the first beam portion striking the second mirror at the second spot, the method comprising: retroreflecting the second beam portion back to the beam splitter along a return path in a second direction opposed to the first direction; splitting the retroreflected second beam portion into a beam travelling from the beam splitter to the second spot and a third beam portion continuing to travel in the second direction; and aligning the split retroreflected second beam portion to be incident on the second mirror at the second spot.
 11. The method as in claim 10 further comprising causing a portion of the third beam portion to strike a fluorescent screen to create an image, and aligning the retroreflector based at least in part on determining whether the image has a predetermined characteristic.
 12. The method as in claim 11 wherein causing at least a portion of the third beam portion to strike a fluorescent screen to create an image comprises reflecting the at least a portion of the third beam portion using a reflective element with an aperture onto the fluorescent screen.
 13. The method as in claim 11 wherein determining whether the image has a predetermined characteristic comprises determining whether the image includes light reflected symmetrically from a rim of the aperture. 